i'm new to here ..i want to know what is the bitcoin core and do we need to download all of it ?

Bitcoin Core may be said as the official wallet for bitcoin. But to use it one should download the the entire blockchain on their systems which requires quite a few Gb's of space so might not be feasible for all. Instead you can use other wallets like Electrum, Multibit, etc. that are light on storage and serve all the same features.